/*
 * Copyright (c) 1997-2018 Oracle and/or its affiliates. All rights reserved.
 * Copyright 2004 The Apache Software Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package cn.taketoday.expression;

import java.io.Serializable;

/**
 * Base class for the expression subclasses {@link ValueExpression} and
 * {@link MethodExpression}, implementing characteristics common to both.
 *
 * <p>
 * All expressions must implement the <code>equals()</code> and
 * <code>hashCode()</code> methods so that two expressions can be compared for
 * equality. They are redefined abstract in this class to force their
 * implementation in subclasses.
 * </p>
 *
 * <p>
 * All expressions must also be <code>Serializable</code> so that they can be
 * saved and restored.
 * </p>
 *
 * <p>
 * <code>Expression</code>s are also designed to be immutable so that only one
 * instance needs to be created for any given expression String /
 * {@link FunctionMapper}. This allows a container to pre-create expressions and
 * not have to re-parse them each time they are evaluated.
 * </p>
 *
 * @since JSP 2.1
 */
@SuppressWarnings("serial")
public abstract class Expression implements Serializable {
  // Debugging

  /**
   * Returns the original String used to create this <code>Expression</code>,
   * unmodified.
   *
   * <p>
   * This is used for debugging purposes but also for the purposes of comparison
   * (e.g. to ensure the expression in a configuration file has not changed).
   * </p>
   *
   * <p>
   * This method does not provide sufficient information to re-create an
   * expression. Two different expressions can have exactly the same expression
   * string but different function mappings. Serialization should be used to save
   * and restore the state of an <code>Expression</code>.
   * </p>
   *
   * @return The original expression String.
   */
  public abstract String getExpressionString();

  // Comparison

  /**
   * Determines whether the specified object is equal to this
   * <code>Expression</code>.
   *
   * <p>
   * The result is <code>true</code> if and only if the argument is not
   * <code>null</code>, is an <code>Expression</code> object that is the of the
   * same type (<code>ValueExpression</code> or <code>MethodExpression</code>),
   * and has an identical parsed representation.
   * </p>
   *
   * <p>
   * Note that two expressions can be equal if their expression Strings are
   * different. For example, <code>${fn1:foo()}</code> and
   * <code>${fn2:foo()}</code> are equal if their corresponding
   * <code>FunctionMapper</code>s mapped <code>fn1:foo</code> and
   * <code>fn2:foo</code> to the same method.
   * </p>
   *
   * @param obj
   *         the <code>Object</code> to test for equality.
   *
   * @return <code>true</code> if <code>obj</code> equals this
   * <code>Expression</code>; <code>false</code> otherwise.
   *
   * @see java.util.Hashtable
   * @see java.lang.Object#equals(java.lang.Object)
   */
  public abstract boolean equals(Object obj);

  /**
   * Returns the hash code for this <code>Expression</code>.
   *
   * <p>
   * See the note in the {@link #equals} method on how two expressions can be
   * equal if their expression Strings are different. Recall that if two objects
   * are equal according to the <code>equals(Object)</code> method, then calling
   * the <code>hashCode</code> method on each of the two objects must produce the
   * same integer result. Implementations must take special note and implement
   * <code>hashCode</code> correctly.
   * </p>
   *
   * @return The hash code for this <code>Expression</code>.
   *
   * @see #equals
   * @see java.util.Hashtable
   * @see java.lang.Object#hashCode()
   */
  public abstract int hashCode();

  /**
   * Returns whether this expression was created from only literal text.
   *
   * <p>
   * This method must return <code>true</code> if and only if the expression
   * string this expression was created from contained no unescaped EL delimeters
   * (<code>${...}</code> or <code>#{...}</code>).
   * </p>
   *
   * @return <code>true</code> if this expression was created from only literal
   * text; <code>false</code> otherwise.
   */
  public abstract boolean isLiteralText();
}
